Formation of thermomechanical interconnection stresses in a high-end portable product

Thermomechanical reliability of electronics has commonly been studied by employing the standard temperature cycling tests. However, due to the localized heating in modern consumer devices, power cycling can be considered a more realistic testing alternative. To better understand the correlation between the thermomechanical loadings caused by the accelerated tests and the real-life operation, a high-end portable device was used to study the stresses and strains induced during both test conditions. Power cycling was executed by running the device's built-in functions to closely represent the typical use. The thermal response was measured with the thermal camera and the displacements quantified with the optical Topography and Deformation Measurement system (TDM). Deformation measurements were also carried out in power cycling at the elevated temperature of 40degC and temperature cycling from room temperature up to 40degC, 60degC, 80degC and 100degC. The induced stresses to interconnections were calculated from the measured displacements with the help of the finite element method. The TDM measurements showed considerable warpage of the printed wiring board during all the tests. The stress and strain fields within component interconnections were not uniform due to the asymmetric design of the device. However, the relative stress distributions were similar in both power and thermal cycling, implying that the method of heating does not markedly influence the stress distribution. In addition, it was found out that the device casing and mounting of the circuit board significantly affect the local strains. The observed stresses and strains serve as a valuable guide in the further development of more realistic and comprehensive reliability testing methods.